Synthesis and thermal evolution of TiO₂-RuO₂ xerogels

Abstract

Gels of the Ti$_{1-x}$Ru$_{x}$O$_{2}$ system, where $x= 0$, 0.001, 0.01, 0.02, 0.05, 0.1, 0.15, 0.2, 0.3 and 0.5 (mol), have been synthesized by a polymeric sol-gel route from Ti (IV)-iso-propoxide and Ru (III) acetyl-acetonate (acac). The mechanisms of the hydrolysis and polycondensation reactions were studied by using Fourier Transform Infrared Spectroscopy (FTIR).

The evolution of the xerogels as a function of temperature was also determined. At temperatures, as low as 200°C, mixtures of antase Ti$_{1-y}$Ru$_y$O$_2$ (Ass) solid solution and rutile Ti$_{1-z}$Ru$_{z}$O$_{2}$ solid solution (Rss) were attained for compositions with $x \le$ 0.3. For $x = 0$, only the anatase phase is present (A) and for $x = 0.5$, mixtures of anatase Ti$_{1 -y}$Ru$_{y}$O$_{2}$ (Ass) solid solution, rutile Ti$_{1-z}$Ru$_{z}$O$_{2}$ solid solution (Rss) and Ru$_{1-a}$Ti$_{a}$O$_{2}$ (RuO$_{2}$ss) solid solution were attained. RuO$_{2}$ catalyzes the anatase to rutile transformation, even at RuO$_{2}$ contents as low as 0.001 mol. Although, from 300 to 400°C the solid solubility of RuO$_{2}$ into rutile-TiO$_{2}$ phase is located at $x\le 0.3$, from 500°C that value is located in the 0.05 $\le x < 0.1$ range. This fact could be due to the metastability of the rutile solid solutions containing ruthenium oxide above 400°C.

According to semiquantitative transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX) analyses, at 700°C, there are compositional variations in both solid solutions, Rss and RuO$_{2}$ss. Thus, the system is chemically heterogeneous. The amount of Ti ions hosted into the RuO$_{2}$ lattice in the solid solution is lower than that of Ru ions into the rutile-TiO$_{2}$ lattice. At this temperature, the contents of these solid solutions are $\approx\!\!17.3$ mol% RuO$_{2}$ into the TiO$_{2}$ lattice (the maximum value found) and around 8.0 mol% TiO$_{2}$ (the maximum value found) into RuO$_{2}$. The RuO$_{2}$ volatilization can promote the segregation of the ruthenium oxide giving rise to the heterogeneity and the metastability observed in this system.